Two-dimensional nanomaterials have emerged as promising materials in field-effect transistors, structurally reinforced composites, chemical/biological sensors, and transparent conductors. Among them, monolayer graphene, a two-dimensional single-atomic layer of carbon, has generated considerable attention as a result of its outstanding electronic, mechanical, and chemical properties. Since its discovery in 2004, monolayer graphene has demonstrated an array of impressive properties, such as charge carrier mobilities in excess of 10,000 cm2V−1s−1 (see Zhang et al., Nature, 438: 201 (2005)), the quantum Hall effect at room temperature (see Novoselov et al., Science, 315: 1379 (2007)), and a variable band gap depending on adsorbate coverage (see Ohta et al., Science, 313: 951 (2006)). Other two-dimensional nanomaterials also have been studied, including those derived from boron nitride (BN), transition metal dichalcogenides, graphite oxide, and the high temperature superconductor Bi2Sr2CaCu2Ox. Further, the semiconducting properties of single flakes of molybdenum disulfide (MoS2) and graphene oxide have been exploited in field-effect transistors.
Despite the technological potential of two-dimensional nanomaterials, methods of synthesizing and purifying them are in their infancy. The most common of these methods, known as micromechanical cleavage, involves drawing a layered crystallite such as graphite over a substrate of interest leaving thin crystallites on the surface (see Novoselov et al., Proc. Nat. Acad. Sci. U.S.A., 102: 10451 (2005)). Although micromechanical cleavage can produce samples of high crystal quality, it has several disadvantages. First, it is difficult to control the position at which crystallites will be placed; consequently, considerable effort is required to locate them. Second, cleavage does not provide control over the thickness of flakes produced, resulting in a limited number of atomically thin crystallites with the majority being tens of nanometers thick. Third, the prospects for large scale production of graphene and other crystallites through micromechanical cleavage are unfavorable.
Other groups have studied epitaxial growth of two-dimensional nanomaterials on various substrates. For example, graphene can be grown directly on metal surfaces or through thermal decomposition of SiC (see Berger et al., Science, 312: 1191 (2006)). However, while these epitaxial graphene samples have the benefit of spanning large areas, full control over the thickness of the resulting crystallites remains a challenge. Moreover, because epitaxial synthesis can occur only on suitable growth substrates, methods of transferring crystallites to other substrates are required for practical applications.
Solution-based methods represent a third route to two-dimensional nanomaterials and can offer several significant advantages over the two approaches described above. First, the desired two-dimensional nanomaterials often can be generated from inexpensive and readily available starting materials. Second, solution-phase techniques do not require transfer from the growth substrate, and can employ existing technologies for scaling up to large volume processing. Several of these methods involve intercalation of graphite and transition metal dichalcogenide crystallites followed by sonication or rapid heating to generate thin flakes (see Viculis et al., J. Mater. Chem., 15: 927 (2005); Yu et al., J. Phys. Chem. C, 111: 7565 (2007); and Joensen et al., Mater. Res. Bull., 21: 457 (1986)). Yet, because of the often violent reactions between the intercalation compounds and water or other solvents, the resulting crystallites usually are at least partially oxidized or otherwise have defect sites which impair their properties (see Li et al., Science, 319, 1229 (2008)).
With respect to the production of monolayer graphene, several groups have explored exfoliating functionalized graphite, such as graphite oxide and graphite fluoride. Unlike pristine graphite, graphite oxide is hydrophilic, and individual graphene oxide layers can be dispersed readily into water. However, graphene oxide is insulating. To regain their electrical conductivity, the exfoliated functionalized graphite materials must be chemically reduced. Despite these treatments, the electronic properties of reduced graphene oxide remain different from those of pristine graphene (see e.g., Tung et al., Nature Nanotech., 4: 25 (2009)). Furthermore, while it is possible to generate monolayer graphene oxide by ensuring that graphite is sufficiently oxidized, it is unlikely that bilayer graphene oxide (or n-layer graphene oxide) could be formed preferentially through controlled oxidation. Graphene nanomaterials have diverse properties depending on the number of layers. For example, monolayer graphene is a 0 eV bandgap semiconductor. Bilayer graphene, on the other hand, has been shown to have a tunable bandgap in the infrared. Meanwhile, trilayer graphene is a semimetal whose band overlap can be controlled by an applied electric field.
Accordingly, there is a need in the art for methods of preparing and purifying two-dimensional nanomaterials with controlled number of layers. In particular, there is a need in the art for methods of preparing and purifying graphene nanomaterials with controlled number of layers, including methods of preparing monolayer graphene and isolating it from other graphene nanomaterials having two or more layers. In addition, there is a need in the art for methods that enable effective dispersion of graphite or graphene in a fluid medium, particularly in water, such that the dispersion can remain stable for an extended period of time.